1. Field of the Invention
The present invention concerns: a method for correcting an EKG signal, acquired during an image acquisition with a magnetic resonance (MR) device according to a magnetic resonance sequence, for interference signals generated by magnetic gradient field jumps that occur during the magnetic resonance sequence, as well as an EKG trigger device for a magnetic resonance measurement.
2. Description of the Prior Art
Magnetic resonance imaging is an established imaging modality that is used especially often in medicine. In many applications, it is the case that the image acquisition must be synchronized with physiological processes, either in order to achieve an optimal image quality or to allow an evaluation of the acquired magnetic resonance data. With regard to cardiac imaging, it is necessary to synchronize the image acquisition with the beating heart. For this purpose, it is typical to measure the EKG signal of the patient via an EKG measurement device and to detect the R-spike in real time. When the R-spike of the EKG signal has been detected, a trigger signal is sent to the running magnetic resonance sequence. For use within the framework of the evaluation of the acquired magnetic resonance data, this trigger signal is stored with the magnetic resonance data but can also be used in order to start defined imaging processes or portions of magnetic resonance sequences (the readout, for example).
However, within the scope of magnetic resonance measurement problems occur in the measurement of the EKG signal because two sources exist that generate unwanted interference signals. One type of such signals is oscillating signals that originate from electrically conductive blood flow (magneto-hydrodynamic effect). Additionally, spike-like signals occur due to currents that are induced due to variations of the gradient fields in the electrical circuits that are created by the patient and the EKG wires. Such chronological variations of gradient fields (gradient ramps, for example) that are defined by the magnetic resonance sequence are generally designated as gradient jumps (discontinuities) in the following. In some sequences, interference signals due to gradient jumps can be strong enough to overlap the actual EKG signal, such that the detection algorithm fails.
In order to enable a robust triggering, the R-spike must also be reliably detectable in the presence of these artifacts. A few solutions have been proposed for this purpose.
A first approach proposes to use alternative trigger mechanisms in addition to EKG triggering, for example optical pulse triggering or acoustic triggering which, however, disadvantageously require additional hardware. In addition, further disadvantages exist in comparison to EKG measurement, in particular a method-related, non-negligible delay from the R-spike peak to the generated trigger signal, which makes this method unsuitable for high-quality cine-imaging in which the triggering should occur before the beginning of the heart contraction.
The measurement of an EKG signal consequently remains as the standard, clinically proven triggering method. A multitude of developments and solution approaches exist that should make the EKG triggering more robust, in particular highly complex signal filtering and the use of multiple EKG wires in order to measure a vector EKG. In spite of all this research work, methods are known in which the success rate of the method is approximately 95%, for example, which means that—according to this example—a triggering error can currently occur in 5% of patients, which can lead to approximately one failure per day in a cardiac center, for example. A further improvement of a triggering with an EKG signal is consequently desirable in magnetic resonance imaging.
Further improvements in EKG triggering with regard to their robustness can be expected only if application-specific optimizations of the detection algorithms for the R-spike are considered. Clinical standard protocols can be divided into two groups:
1. Static imaging, wherein the spin preparation and the acquisition of a fraction of the cardiac cycle are limited. Also included in this imaging group is perfusion imaging (also called dynamic imaging), in which the measurement of multiple spin-prepared slices is completed in one heartbeat, after which a repetition takes place (for example over 10 seconds at cardiac cycles) in order to monitor a dynamic process, for example the first pass of a contrast agent bolus (“first pass measurement”).
2. Cine-imaging, wherein an acquisition takes place over the entire cardiac cycle in order to generate a four-dimensional image data set (thus a film) of the heart movement. More complex methods are also known, for example the technique known as “tagging”, in which at the beginning the image acquisition is preceded by a spin preparation that follows the trigger signal.
Static imaging methods can use a trigger signal that is determined with a considerable delay in comparison to the peak of the R-spike. In order to improve the imaging for this family of applications, it has been proposed to detect the entire R-spike instead of only its rising edge. This method increases the available information about the shape of the signal, such that the R-spike can be clearly differentiated from interference signals in the EKG signal. In this method, however, a considerable delay also occurs from the peak of the R-spike to the point in time of the trigger signal, such that it is unsuitable for cine-imaging.
An additional known way to improve the robustness of a triggering is to monitor the gradient activity of the running magnetic resonance sequence in order to identify and eliminate gradient spikes in the EKG signal. For this procedure it is necessary to measure the gradient activity and to make it accessible to the analysis algorithm for the EKG signal. For example, such a measurement can be achieved by dedicated pick-up coils or be transferred immediately from the gradient control unit to the EKG trigger device. This external gradient activity information must be converted into artifacts in the EKG signal in order to enable an elimination of the interference signals. This multi-parameter conversion requires a pre-calibration, for example by monitoring reference gradient activity. A high cost, and high demands on the electronics implementing the method, consequently are associated with this method.